Rotary magnetron sputtering is well known in the art following McKelvey, AS EVIDENCED BY U.S. Pat. No. 4,446,877. FIGS. 1A, 1B, 2A, and 2B show a conventional, prior art rotary magnetron with a sputter racetrack 2 proximal to the outer surface of the target tube cylinder 1. The target tube cylinder 1 is supported on a backing tube 14. The sputter racetrack 2 is a plasma region proximal to the target material 10 that serves to induce removal of target material 10 from target tube cylinder 1 onto a substrate (not shown). As is known, the sputter target material 10 is formed into a target cylinder 1 and the cylinder 1 is rotated on spindle 6 while an internal magnet bar 20 is held stationary within the cylinder 1, as shown in FIG. 1A. The result is a stationary sputter magnetron racetrack 2 of deposition plasma appears on the rotating tube during operation. FIG. 1B shows a section view of the prior art rotary magnetron from FIG. 1A. Magnet bar 20 is held stationary inside target tube cylinder 1. Magnet bar 20 is comprised of a row of center magnets 28, rows of outer magnets 25 and shunt 26. These magnets are configured with opposite polarities facing outward so that arching magnetic field lines 30 pass from the center magnets 28 to outer magnets 25, penetrating the target and confining plasma 4 on the outside of target 10 surface. The arrows depict the magnetic polarity according to common convention with the arrowhead pointing towards a north pole. At the ends of magnet bar 20, The polarity of the outer magnets 25 is continued by magnets 27 as shown in FIG. 1C (invisible in FIG. 1B). The combination of outer magnets 25 and end magnets 27 create a closed loop around center magnets 28 and an endless racetrack of plasma 2 on the target surface 15. The center magnets 28 can be a single row or multiple rows of magnets as depicted in FIG. 2 of U.S. Pat. No. 5,364,518 (Hartig)
As is known, while considerably better than planar magnetron sputtering, the best target utilization is only approximately 60% for a typical rotary magnetron. The reason for this is relatively large erosion rates of the cylinder 1 in the vicinity of the turnaround 3 of the sputter racetrack 2 compared to the proximal section of the target 12. The target cylinder 1 before usage initial target thickness 15 as shown in FIG. 1A. Now referring to FIG. 2A, target tube material 10 is sputtered off the target cylinder 1, the cylinder section 13 proximal to the turnaround portion 3 of the racetrack 2 erodes faster than the cylinder section 12 proximal to a straightway portion of the racetrack 2. The operational lifetime of a target tube cylinder 1 is exhausted when the target material erodes through to the backing tube 14 at the cylinder section 13. Sputtering of the backing tube 14 onto the substrate contaminates the deposited film. Due to the faster wear, this occurs at a cylinder section 13 first, leaving considerable target material 10 unusable along straight away section 12 underlying straight portion 4. A conventional solution to this detrimental turnaround wear pattern is the usage of a target with added thickness in the end regions and underlying the turnaround portions 3. Such targets are commonly referred to as “dog-boned”. While the thicker dog-boned region improves target utilization, this comes are the cost of more complicated target formation and a larger overall target diameter to the target.
FIG. 2A shows a longitudinal cross-sectional view proximal to the turnaround portion 3 of a conventional, prior art rotary magnetron along line IIA-IIA of FIG. 1A. In this cross-sectional view the internal, stationary magnet bar 20 is shown positioned proximal to rotating backing tube 14 and target material 10. As described above the configuration of magnets 27 and 28 and shunt 26 result in magnetic field lines 30 that arch over and through target material 10. The apex of the field lines arching over and through the target is plotted as line 29. Since, as is known, electrons tend to have concentrated density at the center of the arch, the principal erosion region of cylinder section 13 coincides with line 29 when plasma racetrack 2 is operating. As shown in FIG. 2A, line 29 is roughly perpendicular to the surface of the cylinder 1 in prior art rotary magnetrons. The erosion zone at the at the cylinder portion 13 then is continuously focused over the same linear location at point 7 on the target tube and excessive erosion occurs as shown by the trench in the target profile of cylinder section 13. As shown, the cylinder section 13 is eroded to the backing tube 14 at point 7 while substantial target material remains unused along straightaway cylinder section 12, underlying straight portion 4.
Prior art attempts have been made to improve target utilization have met with limited success. One such prior art configuration is depicted in FIG. 3 and teaches away from the present invention. Like numerals used in FIG. 3 have the meaning ascribed thereto with respect to the preceding figures. FIG. 3 shows a cross-sectional view of U.S. Pat. No. 5,364,518. In this patent, the problem of poor target utilization of rotary magnetrons is recognized and the patent attempts to improve target utilization. FIG. 3 is based on FIG. 7B of U.S. Pat. No. 5,364,518. As shown in FIG. 3 a magnetic shunt 120 is added to the side of magnet 107 and shunt 106 to pull magnetic flux toward the end 125 of target tube cylinder 1. This is taught in U.S. Pat. No. 5,364,518 to widen the target erosion region at the turnaround region 13 and improve target utilization. An analysis of the proposed solution shows the apex of the resulting magnetic field lines 130 plotted as line 109. As shown, line 109 is off normal line 131 by angle θ, also referenced as 110. This geometry results in the erosion zone 114 starting out closer to the end of target cylinder 1. As the target cylinder 1 is eroded, the erosion zone follows line 109 and moves away from the end 125 of the target cylinder 1. Unfortunately, this has only a minimal benefit to overall target utilization. By moving the erosion zone 114 progressively toward the straight away section of cylinder 1 and underlying straight portion 4 of racetrack 2, the erosion zone moves toward a high erosion region of the target and merely broadens the width of the erosion zone relative to that of FIG. 2A. The resulting target erosion profile caused by moving the principal erosion region inward is overlaid the target 10 in FIG. 3.
This broadening is understood with reference to the following equation that approximates the terminal target cross section, t(l) when no further target sputtering can occur without risk of backing tube sputtering:t(l)=Df(l−ek(l−1)f)2  (I)where Df is the final erosion depth and roughly models the width of the erosion zone with smaller value of Df corresponding to a wider erosion zone, is the lateral position and lf is the maximal erosion point denoted at 7 in the aforementioned drawings, and k is a fitting constant.
The approximate fit of equation (I) onto a conventional erosion profile of FIG. 2A is shown graphically as a dashed line in FIG. 2B. For a normalized erosion profile where the initial target thickness is a unit-less value of 1 and point 7 is at l=1, the depicted fit corresponds to two parameter fit for Df=0.48 and lf=0.88, where k=1. It is appreciated that the erosion profile of FIG. 3 is similarly fit with this expression with a best two parameters for Df=0.40 and lf=0.96, where k=1.
Thus, there exists a need for a magnet bar and an apparatus including the same that provides more efficient target utilization for rotary magnetrons. There further exists a need for moving the erosion zone away from the straightaway region of a proximal racetrack to afford an improvement in target utilization.